1. Technical Field
The present invention relates generally to antenna for vehicles and, more particularly, to an antenna for vehicles, in which Automatic Gain Control (AGC) function is improved, thus effectively controlling not only signals in a strong electric field area, but also signals in a weak electric field area close to the strong electric field area.
2. Related Art
With the development of technologies, the desire for entertainment using high-technology information and visual media has increased. To meet the desire, vehicle makers have applied Audio/Video (A/V) systems to vehicles. One of the important requirements of the A/V systems for vehicles is high performance and ease of installation.
Vehicle antennas developed to date include a hand-operated or automatic antenna for receiving radio signals, which has a length of around 70 cm and is mounted on a portion of the front or rear fender of a vehicle and a Television (TV) antenna combined with a radio antenna using a hot-wire structure on the rear windshield of a vehicle.
FIG. 1 is a block diagram of a conventional antenna amplifier module.
The conventional antenna amplifier module includes a slope type AGC unit (ACC1) for controlling weak electric field signals close to a strong electric field. The slope-type AGC unit is configured to have a feedback loop in such a way that a signal is detected at an amplifier output stage and then the strength of a signal to be input to an input stage is adjusted based on the strength of the signal at the amplifier output stage. The slope-type AGC unit transmits voltage, output through a detection diode and a comparator, to a Positive-Intrinsic-Negative (PIN) diode based on the strength of the output signal, so that the attenuation of the PIN diode occurs in the range of predetermined magnitudes (for example, an arbitrary value in the range from 0 dB to 10 dB) based on the magnitude of the voltage, thereby controlling the signals of a weak electric field adjacent to a strong electric field.
That is, the slope type AGC unit is configured in such a way as to detect a signal from the output stage of an amplifier and feed it back to an input terminal of the amplifier so that attenuation may occur according to the strength of the signal.
FIG. 2 is a circuit diagram showing the AGC unit of the conventional amplifier module.
The slope-type AGC unit includes a diode 210 for detecting an input signal, and a comparator 220 for comparing an input signal level with a preset signal level. For example, the comparator may be an Operational Amplifier (OPAMP: LM2904).
The operational principle of the slope type AGC unit is briefly described below.
When an input signal which exceeds a threshold range is input, a PIN diode 230, added to a Transistor (TR) input stage, compulsorily attenuates the input signal so that only a signal having a desired level can be input.
A Frequency Modulated (FM) output signal passes through a Schottky Diode 210 which is configured to perform rectification into a Direct Current (DC) level, and then enters a Schmidt trigger comparator 220 which is configured using an LM2904 OPAMP. The comparator 220 compares the DC level of the signal with a reference DC value which will be used as threshold voltage, and then outputs an output signal.
Since a problem may occur in that an output value continuously varies at the level near noise and threshold, the comparator is configured using a Schmidt trigger having positive feedback. The Schmidt trigger comparator can adjust reference voltage which will be used as threshold voltage by adjusting a resistor 240.
FIG. 3 is a graph showing the AGC operational characteristics of the above-described AGC unit.
When the strength of an output signal increases, voltage over the PIN diode 230 of FIG. 2 increases. Therefore, the attenuation of the PIN diode increases, so that a slope-type operational characteristic appears. The slope type, in which attenuation gradually occurs when a level which is equal to or higher than a predetermined signal enters in the form of feedback of the output signal, is an AGC type which is suitable for controlling the signals of a weak electric field adjacent to a strong electric field.
However, since the strength of a signal is determined at an amplifier output stage, the conventional AGC circuit has problems in that the operation of an AGC unit may be affected by noise depending on an amplifier amplification degree, in that it is not suitable for controlling strong electric field signals, and in that amplification output which exceeds the P1 dB of a TR may occur when a signal, the degree of which is equal to or higher than the amplifier amplification degree, is input.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.